Scanning Near Field Thermoelastic Acoustic Holography (SNFTAH)

ABSTRACT

A system and method for analyzing a sample is described. The system may include, for example, a light source and a scanning probe microscope probe. The light source may generate a coherent laser beam that is modulated by a waveform of a lower frequency. The modulated laser beam is absorbed by the sample causing thermally induced expansion and resulting in an excitation of acoustic waves. The probe is locally deployed near the sample and detects, in real time, perturbations in the excited acoustic waves to detect surface and buried structures of the sample.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application Ser. No. 12/886,929, filed Sep. 21, 2010, which is a continuation of U.S. patent application Ser. No. 12/244,406, filed Oct. 2, 2008, now U.S. Pat. No. 7,798,001, which is a continuation of U.S. patent application Ser. No. 11/244,747, filed Oct. 6, 2005, now U.S. Pat. No. 7,448,269, which is a continuation-in-part of U.S. patent application Ser. No. 10/913,086, filed Aug. 6, 2004, which claims benefit from and priority to U.S. Provisional Application No. 60/494,532, filed Aug. 12, 2003. The above-identified applications are hereby incorporated herein by reference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant Number DB1-0649916/001 awarded by the National Science Foundation. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

Known acoustic microscopes are used for imaging structures such as integrated circuit (IC) structures. The spatial resolution, w, of an acoustic microscope is given by:

$w = {0.51\frac{\vartheta}{f \cdot {NA}}}$

where θ is the speed of sound in the coupling medium, f is the frequency of the acoustic/ultrasonic wave, and N.A. is the numerical aperture of the lens. For a frequency of 1 GHz, the nominal spatial resolution attainable is approximately 1.5 μm. Further, the acoustic microscope has two other major roadblocks in getting high resolution: (1) impedance mismatches and coupling fluid attenuation that is proportional to f². Higher resolution alternatives for nondestructive mechanical imaging include the atomic force microscope (AFM) or scanning probe microscope (SPM) platforms. A few examples include: force modulation microscopy (FMM) as described by P. Maivald, H. J. Butt, S. A. C. Gould, C. B. Prater, B. Drake, J. A. Gurley, V. B. Elings, and P. K. Hansma in Nanotechnology 2, 103 (1991); ultrasonic-AFM as described by U. Rabe and W. Arnold in Appl. Phys. Lett. 64, 1423 (1994); and ultrasonic force microscopy (UFM) as described by O. V. Kolosov, K. Yamanaka in Jpn. J. Appl. Phys. 32, 1095 (1993); by G. S. Shekhawat, O. V. Kolosov, G. A. D. Briggs, E. O. Shaffer, S. Martin and R. Geer in Nanoscale Elastic Imaging of Aluminum/Low-k Dielectric Interconnect Structures, presented at the Material Research Society, Symposium D, April 2000 and published in Materials Research Society Symposium Proceedings, Vol. 612 (2001) pp. 1.; by G. S. Shekhawat, G. A. D. Briggs, O. V. Kolosov, and R. E. Geer in Nanoscale elastic imaging and mechanical modulus measurements of aluminum/low-k dielectric interconnect structures, Proceedings of the International Conference on Characterization and Metrology for ULSI Technology, AIP Conference Proceedings. (2001) pp. 449; by G. S. Shekhawat, O. V. Kolosov, G. A. D. Briggs, E. O. Shaffer, S. J. Martin, R. E. Geer in Proceedings of the IEEE International Interconnect Technology Conference, 96-98, 2000; by K. Yamanaka and H. Ogiao in Applied Physics Letters 64 (2), 1994; by K. Yamanaka, Y. Maruyama, T. Tsuji in Applied Physics Letters 78 (13), 2001; and by K. B. Crozier, G. G. Yaralioglu, F. L. Degertekin, J. D. Adams, S. C. Minne, and C. F. Quate in Applied Physics Letters 76 (14), 2000. Each of these techniques is traditionally sensitive to the static elastic properties of the sample surface.

Recent developments in atomic force microscopes have involved the application of ultrasonic frequency (MHz) vibrations to the sample under study and non-linearly detecting of the deflection amplitude of the tip at the same high frequencies. With this arrangement, which is commonly identified as an ultrasonic force microscope, the ultrasonic frequencies employed are much higher than the resonant frequency of the microscope cantilever. The microscope exploits the strongly non-linear dependence of the atomic force on the distance between the tip and the sample surface. Due to this non-linearity, when the surface of the sample is excited by an ultrasonic wave, the contact between the tip and the surface rectifies the ultrasonic vibration, with the cantilever on which the tip is mounted being dynamically rigid to the ultrasonic vibration. The ultrasonic force microscope enables the imaging and mapping of the dynamic surface viscoelastic properties of a sample and hence elastic and adhesion phenomenon as well as local material composition which otherwise would not be visible using standard techniques at nanoscale resolution.

The drawback of ultrasonic microscopy is that it measures only the amplitude due to ultrasonically induced cantilever vibrations. Moreover, where the sample is particularly thick and has a very irregular surface or high ultrasonic attenuation, only low surface vibration amplitude may be generated. In such circumstances the amplitude of vibration may be below the sensitivity threshold of the microscope in which case measurement is impossible. Moreover, none of the above mentioned techniques measures with high resolution the acoustic phase, which is very sensitive to subsurface elastic imaging and deep defects identification which are lying underneath the surface, without doing any cross sectioning of the samples.

Out-of-plane vibrations created by non-linear tip sample interaction make a very hard elastic contact with the sample surface. Ultrasonic force microscopy (UFM) uses the same method except for a amplitude component rather than a phase contrast. If non-linearity is present in the system, most of the phase contrast will come from the surface and not from a surface/sub-surface phase contrast. Additionally, non-linear tip sample interaction may not provide results for soft materials. Furthermore, in UFM, high mechanical contrast may be acquired with little sub-surface contrast.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a high spatial resolution phase-sensitive technique, which employs a scanning near field ultrasonic holography methodology for imaging buried or other subsurface structures or variation in the specimen. Scanning near field ultrasound holography (SNFUH) uses a near-field approach to measure time-resolved variations in ultrasonic oscillations at a sample surface. As such, it overcomes the spatial resolution limitations of conventional phase-resolved acoustic microscopy (i.e. holography) by eliminating the need for far-field acoustic lenses.

The fundamental static and dynamic nanomechanical imaging modes for the instrument of the present invention are based on nanoscale viscoelastic surface and subsurface (e.g., buried nanostructure) imaging using two-frequency ultrasonic holography. The scanning near-field ultrasonic technique of the present invention vibrates both the cantilevered tip and the sample at ultrasonic/microwave frequencies. The contact, soft-contact and near-contact modes of tip-sample interaction enable the extraction of the surface acoustic waves signal between the two ultrasonic vibrations.

Perturbations to the phase and amplitude of the surface standing acoustic wave may be locally monitored by the SPM acoustic antenna via lock-in and SNFUH electronic module. As the specimen acoustic wave gets perturbed by buried features, the resultant alteration in the surface acoustic standing wave, especially its phase, is effectively monitored by the SPM cantilever. Thus, within the near-field regime (which enjoys superb spatial resolution), the acoustic wave (which is non-destructive and sensitive to mechanical/elastic variation along its path) is fully analyzed, point-by-point, by the SPM acoustic antenna in terms of its phase and amplitude. Thus, as the specimen is scanned across, a pictorial representation of specimen acoustic wave's perturbation is recorded and displayed, to offer quantitative account of the internal features of the specimen.

Certain embodiments provide contact, soft (e.g., intermittent) contact, and/or near contact modes of operation to identify surface and subsurface (e.g., buried) characteristics of a specimen. Additionally, an SNFUH electronic module extracts surface acoustic phase and amplitude with or without non-linear tip sample interaction.

Some embodiments according to the present invention provide that laser beam radiation is focused on a sample surface. As the laser beam radiation is absorbed by the sample surface, thermally induced expansion of the sample occurs which generates thermo acoustic waves through thermal waves. The carrier wave can be modulated with a very low frequency modulated waveform (e.g., a frequency of approximately 20 KHz to approximately 200 KHz). The amplitude, for example, of the modulated waveform riding on the carrier may change depending on the defects close to the sample surface or surface mechanical variation. A sharp SPM cantilever may be used to monitor the modulated surface acoustic wave to identify buried defects and/or to sample surface nanomechanical properties with very high spatial resolution. The amplitude signal, for example, may be fed into a lock-in amplifier and an output of the lock-in amplifier may be used to generate amplitude scanning near field thermoelastic acoustic holography (SNFTAH) images. Acoustic transducers and coupling solvents are not employed in some ultrasound holography methods according to some embodiments of the present invention. The frequency of the laser beam can be easily tuned from approximately a megahertz to approximately a gigahertz to attain diffraction limit free images with very high spatial resolution.

Some embodiments according to the present invention provide that amplitude and/or phase of the SNFTAH signal may be detected which can provide spatial as well as depth resolution of any buried defects. For example, an approximately 1 GHz excitation may provide resolution to identify an approximately 1 nm defect underneath the sample surface. In some embodiments according to the present invention, laser excitation is modulated by a waveform with a frequency of, for example, approximately 20 KHz to approximately 200 KHz. When the modulated laser excitation is focused on sample surface, thermo acoustic waves are generated. A small frequency ultrasonic transducer (e.g., providing oscillation in a range from approximately 20 KHz to approximately 200 KHz) may be placed on the cantilever to launch ultrasonic waves of low frequency. In some embodiments according to the present invention, the launched ultrasonic waves interfere with the modulated waveforms to form beats on the surface of the sample. Since the cantilevers may have a lower resonance frequency, the cantilever may easily monitor the surface perturbations which results from delay in the acoustic phase due to buried defects underneath the surface. The phase resolution may be dramatically increased with the excitation frequency due to the reduction and/or elimination of diffraction effects. Some embodiments according to the present invention provide that a waveform oscillator drive the cantilever with a waveform having a frequency in the range of approximately 20 KHz to approximately 200 KHz and that a lock-in amplifier in combination with a mixer circuit determines the amplitude and/or phase of the beats via an optical detection method (e.g., a conventional optical detection method). The phase and/or amplitude components may be plotted into an auxiliary channel of an SPM system to generate an image of sub-surface defects, for example.

These and other advantages and novel features of the present invention, as well as details of an illustrated embodiment thereof, will be more fully understood from the following description and drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a block diagram illustrating the scanning probe microscope with scanning near field ultrasound holography of the present invention;

FIG. 2 is an illustration of atomic force microscopy of the present invention with a vibrating cantilever tip and vibrating sample;

FIG. 3(A) is a schematic illustration of a model nanoparticle system for validation of SNFUH.

FIG. 3(B) shows an AFM (topography) image with a featureless top polymer surface.

FIG. 3(C) shows the phase image of SNFUH revealing buried gold nanoparticles with high definition.

FIG. 4(A) shows a schematic of a model test sample for detecting embedded defects/voiding in shallow trenches.

FIG. 4(B) shows an AFM (topography) image with a uniform coating of dielectric material.

FIG. 4(C) shows a phase image of SNFUH that reveals the surface elastic contrast and embedded voiding in polymer coating over nitride and hardening of the coating at the trench walls.

FIG. 4(D) shows a line profile across a void marked across X-Y.

FIG. 5(A) shows an AFM topography of malaria-infected red blood cells.

FIG. 5(B) shows an SNFUH phase image from malaria-infected red blood cells.

FIG. 5(C) represents an AFM topography of early-stage incubation of parasite infection in malaria-infected red blood cells.

FIG. 5(D) represents an SNFUH phase image of early-stage incubation of parasite infection in malaria-infected red blood cells.

FIG. 6(A) depicts AFM (topography) imaging of a copper-low K dielectric interconnect system.

FIG. 6(B) depicts SNFUH imaging of a copper-low K dielectric interconnect system.

FIG. 6(C) shows a line profile across the voids in FIG. 6(B).

FIG. 7 shows a feedback control circuit used in accordance with an embodiment of the preset invention.

FIG. 8 illustrates the feedback circuit in the context of an embodiment of an electronic readout system used in accordance with an embodiment of the present invention.

FIG. 9 illustrates a flow diagram for a method for scanning near field acoustic imaging used in accordance with an embodiment of the present invention.

FIG. 10 shows an embodiment of a system for scanning near-field thermoelastic acoustic holography according to the present invention.

FIG. 11 shows another embodiment of a system for scanning near-field thermoelastic acoustic holography according to the present invention.

The foregoing summary, as well as the following detailed description of certain embodiments of the present invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, certain embodiments are shown in the drawings. It should be understood, however, that the present invention is not limited to the arrangements and instrumentality shown in the attached drawings.

DETAILED DESCRIPTION OF THE INVENTION

Certain embodiments of the present invention are directed to a nondestructive, high resolution, sub-surface nanomechanical imaging system. The system is capable of directly and quantitatively imaging the elastic (static) and viscoelastic (dynamic) response of a variety of nanoscale materials and device structures with spatial resolution of a few nanometers depending on the ultrasonic frequencies. For viscoelastic high resolution sub-surface nanomechanical imaging the target maximum probe frequency is around 5-10 GHz, for example. In an embodiment, the maximum relative phase resolution at this frequency is estimated to be 0.001° leading to a viscoelastic time resolution of less than <1 ps. The instrument of certain embodiments of the present invention operates in a manner similar to commercially available scanning probe microscopes (SPMs) in that quantitative, digital, rastered, nanometer-scale images are obtained of the sample elastic modulus, and sample viscoelastic response frequency. The instrument also provides conventional SPM imaging modes including topography, frictional, and force modulation imaging.

Applications for certain embodiments of the present invention are numerous and represent areas of critical need in Molecular electronics, Nanosystems (NEMS), and Nanotechnology, in general. By combining the nanometer-scale spatial resolution of conventional SPMs with the sub-surface defect identification and imaging capabilities of acoustic or ultrasonic microscopes, the instrument fills a critical need in characterizing and investigating the nanomechanics of nanoscale systems. The SNFUH system and method may be used for: (1) in-vitro imaging of biological specimens, tissues and cells, (2) nanomechanical imaging of buried structures, inclusions in nanocomposites, failure analysis in IC structures and devices, (3) mechanical properties of low-K materials, (4) stress variation in 3D structures and interconnects, (5) flaw imaging in ceramics and quantitative evaluation of mechanical properties, etc.

Certain embodiments of the present invention are based on Nanoscale viscoelastic surface and subsurface (e.g., buried nanostructure) imaging using two-frequency ultrasonic holography. This is essentially a ‘scanning near-field’ ultrasound technique, where both the cantilevered tip 10 and the sample 12 are vibrated at ultrasonic/microwave frequencies. Contact and soft-contact tip-sample interaction enables the extraction of surface acoustic wave amplitude and phase with high resolution.

In SNFUH mode, perturbation to the standing surface acoustic wave resulting from specimen acoustic wave scattering is monitored by an SPM acoustic antenna. The resulting cantilever deflection merely follows the perturbation to the surface standing acoustic wave, which represents the dissipative lag/lead in the surface response with respect to the tip reference frequency (i.e. the time of flight delay of the specimen acoustic waves reaching the sample surface). Extracting the spatial dependence of this phase term provides image contrast indicative of the relative elastic response of the buried structures, interfaces, and embedded defects to the specimen acoustic wave and thus the resultant perturbation to the standing surface acoustic wave.

Certain embodiments of the present invention provide a system that measures subsurface (e.g., buried) defects, delaminations; cracks; stress migration and etc., while maintaining the high resolution of the atomic force microscope. It utilizes (1) an atomic force microscopy system having a cantilever 14 with a tip 10 at a free end sitting on top of the vibrating device 16 for supplying vibrations to the cantilever at a frequency greater than cantilever resonance frequency, (2) a sample 12 having a vibration device 18 sitting under it for providing high frequency excitations and (3) an optical detector or other detector for detecting movement of the cantilever. It detects the beats, products, additions frequencies and beats, products of their harmonics and modulated waveforms, when the vibrating tip interacts with the vibrating sample, which falls within its detection range. With this embodiment, it is possible to recover surface acoustic wave phase information of the tip-surface mechanical interaction, which allows measurement of viscoelastic properties and enables the application of acoustic holography algorithms for imaging nanoscale sized sub-surface (e.g., buried) defects. The microscopy apparatus utilizes scanning Near Field Ultrasound Holography (e.g., SNFUH) for high resolution nanomechanical imaging of buried defects and structures.

The surface acoustic wave's amplitude and phase are experimentally extracted from the tip deflection signal via lock-in detection. The phase sensitivity of this measurement is involved in extracting time-resolved mechanical properties of materials as well as potentially enabling subsurface imaging (e.g., buried nanostructures).

Certain embodiments of the present invention detect the phase of transmitted acoustic wave directly at wafer/device surface. Further, certain embodiments of the present invention detect the phase of surface acoustic wave directly at wafer/device surface. Further, certain embodiments of the present invention utilize scanning nanoprobe phase detection so as to eliminate the need for acoustic lenses. The Nanoprobe Acoustic Antenna (AFM Tip) of certain embodiments of the present invention is advantageous because, for example, it provides induction of MHz-GHz nanoprobe mechanical oscillations via high frequency flexural mode excitation, i.e. the mechanical wave guide and the cantilever monitors the phase shifts between tip 10 and sample 12 acoustic/ultrasonic vibrations.

As shown in FIGS. 1 and 2, two oscillations are applied to the tip 10 and sample 12 by two matched piezo crystals 16 and 18 attached to the Si substrate of the tip and the base of the sample, respectively. Each piezo 16, 18 is driven by a separate waveform with a SNFUH electronic module 36 providing the input frequency to an RF lockin amplifier 40 for surface acoustic wave (SAW) amplitude and phase extraction. The SNFUH electronic module 36 selects beats, product, and/or addition frequencies for example, to aid in performing holography in contact, soft-contact, and near-contact modes. Additionally, the SNFUH electronic module 36 allows SNFUH to be performed in the linear regime of tip-sample interaction. In an embodiment, the SNFUH electronic module 36 includes a mixer circuit, variable resistor(s), op-amp(s), band pass filter(s), and/or other filters for mixing frequency signals and selecting frequency products, additions and beats, for example.

Any Scanning Probe Microscope (SPM) may serve as the base platform. A signal access module (SAM) 22 is used as the input site for SNFUH, and modulus-calibration signals. The integrated piezo (for high frequency excitation) will enable ultrasonic excitation of higher-order flexural resonances of the cantilever tip 10 to provide the ultrasonic vibration.

The sample ultrasonic vibration is driven by function generator 32. Second function generator 34 applies the sample ultrasonic vibration. The resulting differential output signal from detector is accessed with the signal access module (SAM) 22 and acts as the input to RF lockin amplifier 30 or similar lockin amplifier for extraction of SAW amplitude and phase. The lockin response signal constitutes a SAW amplitude and phase which act as a input into the signal acquisition electronics 46, via the SAM 22, for image display and analysis. The SNFUH electronic module circuit 36 extracts beats, products of fundamental and harmonics, and/or modulated waveform(s), which serve as a reference for a RF lockin amplifier 40 or other lockin amplifier, for example. The differential output of optical detector (A-B) is input, via the SAM 22, into the RF lockin 40. The resulting output constitutes the SNFUH image signal. A computer 44 or other processor running data acquisition/analysis software, such as Lab View or other data acquisition and/or analysis software, acquires both the A-B signal from the digital scope and the lockin. In an embodiment, a switch may be included to select an SNFUH or UFM signal for acquisition, for example.

In an embodiment, the sample piezo consists of an insulator/electrode/piezo/electrode/insulator blanket multilayer (e.g., 10 cm×10 cm) stack. The insulators consist of epoxied machinable ceramics or thin, spin-cast polymer coatings, dependent upon ultrasonic coupling efficiency. The Cr/Au electrodes or other similar electrodes provide electrical contact between the piezo and the second function generator 34. The assembly is counter-sunk into a modified SPM sample mount.

As shown in FIG. 2, using SNFUH, a high frequency acoustic wave is launched from below the specimen 12, while another high frequency acoustic wave is launched at least a slightly different frequency is launched on the SPM cantilever 10. The SNFUH electronic module 36 is used to spatially monitor the phase perturbation to a standing surface acoustic wave, which results from a scattered specimen acoustic wave. The resonant frequency of the cantilever, f₀, may be in the 10-100 kHz range, for example.

Certain embodiments may also include a feedback circuit, such as the cantilever resonance feedback circuit 50 depicted in FIG. 7. The feedback circuit 50 includes a first op amp (OA) 52, a second op amp (OA) 54, a phase compensator (PC) 56, a voltage-controlled oscillator (VCO) 62, a waveform or function generator 68 and a cantilever tip having a piezo transducer 74 interacting with a sample 78.

For SNFUH operation to be uniquely calibrated across samples, phase of the cantilever may be fixed. In order to fix the phase of the tip, a resonance feedback circuit, said as feedback circuit 50 may be employed. The feedback circuit 50 maintains the tip carrier frequency at resonance and fixes or sets the phase, so that the tip phase is a stable reference for sample phase. For example, at higher frequencies (e.g., 150 MHz-10 GHz), the cantilever easily strays from its resonance, and feedback may be used to maintain the cantilevered tip frequency at resonance. Both the sample and cantilever may maintain their resonance frequencies to generate a high resolution viscoelastic response. In an embodiment, the system may operate in feedback mode with the feedback circuit 50 generating feedback for frequency resonance, or the system may operate without the feedback circuit 50 activated.

In the feedback circuit 50, the voltage-controlled oscillator 62 drives the tip piezo transducer 74. The VCO 62 is connected through the phase compensator 56, which acts as an input to an op amp pair 52, 54 for feedback control. If the cantilever resonance frequency shifts during scanning, the reduction in tip vibration amplitude will reduce the voltage across the piezo transducer on the cantilever. This voltage will cause a shift in the PC 56 output. The shift in PC output will bring the VCO 62 back into resonance.

FIG. 8 illustrates the feedback circuit 50 in the context of an embodiment of an electronic readout system 800 used in accordance with an embodiment of the present invention. The electronic readout system 800 may be a MOSFET embedded electronic readout, for example. Using embedded MOSFET as electronics feedback may provide a current sensitivity of ΔI_(d)/I_(d)=10⁻⁶/nm of cantilever bending. Deflection sensitivity of the electronic readout may be of the same order as optical feedback detection, for example. In an embodiment, deflection sensitivity may be approximately three orders of magnitude higher than existing passive and active detection technologies, such as piezoresistive detection. In an embodiment, a high signal-to-noise ratio and minimal 1/f noise allow the MOSFET embedded electronic readout to be used for electronic feedback in SPM's (scanning probe microscopes), for example.

The feedback circuit 50 may be used to control a power supply 84 which supplies power to a piezo 86. The piezo 86 includes contacts, such as Au (gold) contacts 88, as well as an actuator 90 and a BiMOS transistor 92. The piezo 86 is driven by an oscillator 94. Feedback from the oscillating piezo 86 is gathered by the electronic detection unit 96. The feedback signal from the electronic detection unit 96 is converted using the analog to digital converter (ADC) 98 and fed into the feedback circuit 50 for control of the power supply 84. Set point 100 provides a basis or reference for operation of the feedback circuit 50. Feedback from the circuit 50 helps to ensure that the tip and the sample are being vibrated at their respective resonance frequencies, for example.

An example of viscoelastic nanomechanical imaging is shown in FIG. 3. FIG. 3(A) shows gold nanoparticles dispersed on a polymer coated substrate are buried under an approximately 500 nm thick polymer layer. Use of a model polymer-nanoparticle composite demonstrates the high lateral spatial resolution and depth sensitivity of the SNFUH approach. A specimen consisting of gold nanoparticles buried deep underneath a polymer cover layer was prepared by dispersing colloidal gold nanoparticles on a polymer (poly(2-vinylpyridine)—PVP) coated silicon substrate. The gold nanoparticles have an average diameter of 15 nm and are well dispersed on the film surface. The nanoparticles were then fully covered with another polymer film about 500 nm thick, as shown schematically in FIG. 3(A). The normal AFM topography scan, FIG. 3(B), shows a smooth featureless surface of top polymeric layer with surface roughness of approximately 0.5 nm. On the other hand, the phase image of SNFUH, FIG. 3(C), shows well-dispersed gold nanoparticles buried approximately 500 nm deep from the top surface. The contrast in the phase image of SNFUH arises from the elastic modulus difference between the polymer and gold nanoparticle, which induces the time dependent phase delay of the acoustic waves reaching the sample surface.

To demonstrate the efficacy of SNFUH in identifying underlying defects in narrower trenches, shallow trench structures may be fabricated as shown in FIG. 4(A). The trenches are etched in SOD (spin-on-dielectric) with a 50 nm thin layer of LPCVD Si₃N₄ as a capping layer and then Si₃N₄ is etched down in the 1 μm deep trenches using the wet processing. Trench width in the example is about 400 nm. A 500 nm thick layer of polymer [Benzocyclobutene (BCB)] was spin-coated followed by thermal annealing for curing the polymer.

FIG. 4(A) shows a schematic of series of isolated shallow trench structures. FIG. 4(B) shows a conventional AFM topography image, while FIG. 4(C) is a corresponding (simultaneously recorded) SNFUH phase image. The typical 7.5×7.5 μm² topography scan shows uniform and contiguous polymeric coating on SiN and inside the trenches. On the other hand, the corresponding SNFUH phase image shown in FIG. 4(B) reveals phase contrast reminiscent of embedded voiding within the polymer, and at the SiN-polymer interfaces. The dark contrast in the phase image in polymer coated SiN lines corresponds to voids at polymer-SiN interface, i.e. voiding underneath the contact. The contrast is due to the distinct viscoelastic response from the specimen acoustic wave from the voids, for example. A hardening of the polymer in the trench and its sidewall is also evident in the phase image, which results from thermal annealing and possibly poor adhesion with SOD, for example. FIG. 4(D) shows a line profile of phase across X-Y from FIG. 4(C). A subsurface phase resolution of 50 mdeg may be achieved, for example. Current methods of diagnosis employ destructive approaches such as wet etching followed by SEM imaging, which are undesirable. Thus, SNFUH may be an improved tool-set for such subsurface metrology needs.

The efficacy of SNFUH to imaging embedded or buried sub-structures in biology is demonstrated in FIG. 5, which depicts high resolution and remarkably high contrast arising from malaria parasites inside infected red blood cells (RBCs). FIG. 5 demonstrates early stage direct and real-space in-vitro imaging of the presence of parasites inside RBCs without labels or sectioning of cells, and under physiologically viable conditions. Plasmodium falciparum strain 3D7 was cultured in-vitro by a modification of the method of Haldar et al. Parasites were synchronized to within 4 hours using a combination of Percoll purification and sorbitol treatments, cultured to 10% parasitemia, and harvested at the indicated times, for example.

SNFUH imaging may be performed using the near-contact mode method for imaging soft structures, for example. An SNFUH electronic module may be used to bring the cantilever in near-contact mode and then the sample was subsequently scanned over the RBCs while maintaining the near-field regime. FIGS. 5(A) and 5(B) show AFM topography images and SNFUH phase images from infected RBCs, respectively. The AFM topography image shows the typical surface morphology of infected RBC, while the SNFUH phase image shows high contrast from the parasite residing well inside the RBC. In addition to several other features reminiscent of membrane proteins and sub-cellular contents, multiple parasites are clearly evident. In order to further demonstrate the capability of SNFUH for early stage diagnosis of parasite infection, RBCs incubated for only four hours are examined, which is difficult to validate by other non-invasive technique (e.g., fluorescence tagging). FIGS. 5(C) and 5(D) show a pair of images similar to those in FIGS. 5(A) and 5(B). SNFUH may be sensitive to early stage parasite infection in RBC, as reflected by image contrast consistent with parasite infection, for example.

FIG. 6 shows a series of low-K dielectric polymer and copper lines with lateral dimension of about 200 nm for the polymer and around 60 nm for copper. FIG. 6(A) shows the conventional topography image, while FIG. 6(B) is the corresponding (simultaneously recorded) SNFUH phase image. The typical 1400×1400 nm² topography scan shows uniform and contiguous polymer and copper lines. However, the corresponding SNFUH phase image shown in FIG. 6(B) reveals phase contrast reminiscent of sub-surface voiding in copper lines. FIG. 6(C) shows a line profile across the voids. The dark contrast in the phase image of copper lines corresponds to voids underneath the metal. The presence of this contrast in phase image implies that there is insufficient metal filling at the bottom, i.e. voiding underneath the contact, which undergoes a distinct viscoelastic response. Interestingly, a hardening of the polymeric regions and its sidewall is also evident in the phase image, which results from RIE processing and chemical-mechanical-polishing (CMP). SNFUH may serve as a tool-set for such sub-surface metrology challenges.

Thus, SNFUH may be used to facilitate: (1) quantitative high resolution nanomechanical mapping of subsurface (e.g., buried) structures to identify process-induced mechanical variations and/or nanoscale cohesive defects; (2) nanomechanical viscoelastic (dynamic) imaging to specifically investigate surface and subsurface interfacial adhesive (bonding) response, etc.

Other applications for the system and method of the present invention include: (1) non-destructive imaging of subsurface defects in 3D interconnects and stress migration along the devices due to electrical biasing; (2) non-destructive inspection for interconnect nanotechnology for nanometer-scale resolution, to enable imaging of electro-mechanical defects (e.g. nanotube contacts) and to enable imaging of nanoscale integrity in molecular interconnect assemblies; (3) subsurface nano-cracks, stress, delamination identification in ferroelectrics, ceramics and micromechanical structures and devices; (4) non-destructive defect review and process control in integrated IC materials and devices to provide modulus measurement for soft materials (i.e. porous dielectrics) and to provide void and delamination defect detection to avoid off-line, cross-sectional failure analysis; (5) self assembled monolayers and subsurface defects in biological cells and materials, in-vitro imaging of biological cells, tissues and membranes, nano-bio mechanics and (6) quantitative extraction of elastic modulus with high accuracy.

FIG. 9 illustrates a flow diagram for a method 900 for scanning near field holography imaging used in accordance with an embodiment of the present invention. First, at step 910, a sample is positioned with respect to a cantilever for nanomechanical imaging of the sample. Surface and/or sub-surface imaging may be performed with respect to the sample, such as a tissue or other sample. At step 920, a tip of the cantilever is vibrated at a first frequency. The frequency may be a first microwave, ultrasonic, or other acoustic frequency, for example. Then, at step 930, the sample is vibrated at a second frequency, such as a second microwave, ultrasonic, or acoustic frequency, for example. In an embodiment, the second frequency vibrates the sample at a frequency that is offset from the first frequency vibrating the tip. In an embodiment, the tip and sample piezos are vibrated at their respective resonance frequencies.

Next, at step 940, interaction between the vibrating tip and the vibrating sample is detected. The interaction may be a physical interaction and/or a non-contact signal interaction between the tip and sample, for example. The interaction may constitute movement of the tip, for example. Tip movement may be provided as a tip deflection signal, for example. In an embodiment, the interaction may include a linear and/or non-linear interaction between the tip and the sample. In an embodiment, SNFUH may be performed using the linear tip-sample interaction in soft contact and near contact mode to obtain the high resolution sub-surface phase. SNFUH may be performed in soft and near contact modes to obtain the sub-surface information, such as buried defects or variations.

At step 950, amplitude and phase information associated with the surface acoustic waves of the sample are extracted. Amplitude and phase information may be extracted from the tip deflection signal using lock in detection, for example. In an embodiment, subsurface mechanical data, such as interfacial bonding, regarding the sample may also be extracted from the tip deflection signal.

At step 960, surface and/or subsurface characteristics of the sample may be imaged using the amplitude and phase information. In an embodiment, a spatial variation of surface and subsurface viscoelastic phase may be imaged, for example. In an embodiment, a characteristic viscoelastic response time of the sample may be defined based on the amplitude and phase information. Then, at step 970, vibration of the cantilever tip is maintained at a tip piezo resonance frequency, and vibration of the sample is maintained at a sample resonance frequency. Feedback, such as electrical feedback, may be provided to maintain the tip resonance frequency and the sample resonance frequency.

In an embodiment, product frequencies may be used with optical detection to obtain biological imaging with high subsurface resolution. The sample and cantilever are excited at their fundamental resonance frequencies (e.g., 1.96 MHz and 3.28 MHz, respectively). Additionally, the individual sample and cantilever carriers are modulated with one or more modulation frequencies (e.g., 25 kHz and 35 kHz, respectively). Next, a combination of a SNFUH electronic module and RF lock in band pass filter outputs a product of the two modulated waveforms. The product output is then fed into a RF lock in amplifier reference input.

Using product frequencies allows improved selection of carrier frequencies. In an embodiment, the larger the frequency of acoustic oscillations, the higher the order of phase contrast that may be obtained from SNFUH images. Thus, smaller features not seen at lower carrier frequencies may be detected using a higher frequency carrier. Additionally, use of product frequencies allows use of non-matching tip and cantilever piezos.

In an embodiment, forces between the cantilever and sample may be controlled during SNFUH operation in near contact mode. Contacting the cantilever with the biological samples may rupture the samples. However, near contact operation allows monitoring and subsurface imaging of soft structures. Near contact mode operation may provide sub-surface imaging of soft structures as well as providing quantitative analysis of biological structures, cells and/or tissues, for example.

In an embodiment, beat frequencies may be used to monitor samples in near contact mode. Alternatively, frequency addition may be used for sample monitoring in near contact mode. In an embodiment, harmonics, as well as or in addition to fundamental frequencies, may be used in beat frequency, product frequency, and/or frequency addition (sum) operation. For example, the system may perform optical and/or electronic detection according to a variety of frequency strategies up to 1000 MHz with a very thin film of ZnO.

In an embodiment, cantilever and sample carrier frequencies may be modulated with amplitude modulation. For example, two carriers, a carrier for the cantilever and a carrier for the sample are amplitude modulated individually. In this configuration, the tip-sample assembly may be excited with a higher frequency (with or without matching piezos). Then, an amplitude modulated waveform may be obtained from both the cantilever and sample and input to a SNFUH electronic module. The output of the electronic module is a product/difference/addition frequency. A beat or difference frequency is a difference between modulation frequencies, for example.

In an embodiment, an electronic readout device may be implemented with the sample monitoring system. An example of such a readout device may be the readout device described in U.S. patent application Ser. No. 10/996,274, filed on Nov. 23, 2004, entitled “Method and System for Electronic Detection of Mechanical Perturbations Using BiMOS Readouts”, which is herein incorporated by reference. In an embodiment, using a readout circuit allows product frequencies to be used without modulation since operation is not limited by response time as with optical photodiodes, for example.

In an embodiment, use of electronic detection eliminates optical detection of amplitude and phase removes or eases limitations imposed by a photo-detector response frequency, such as a 1 MHz photo-detector response frequency. Electronic detection aids in fabricating multi-active probes with on-chip integrated piezo-actuator (e.g., ZnO) and embedded MOSFET feedback electronics. Additionally, electronic detection does not limit detection of subsurface features based on beat frequency. Multiples of frequencies may be used to enhance both amplitude and subsurface phase contrast, and thus the viscoelastic response. Enhanced viscoelastic response results in enhanced phase contrast from features less than 50 nm, for example, which may be difficult to detect using only beat frequencies.

Thus, certain embodiments provide a scanning near field ultrasound holography (SNFUH) technique to image high resolution buried nanostructures, defects, 3D tomography, identification of individual layers in multilayer thin film stacks and dopant mapping, for example. Certain embodiments integrate three approaches: a combination of scanning probe microscope platform (which enjoys excellent lateral and vertical resolution) coupled to micro-scale ultrasound source and detection (which facilitates “looking” deeper into structures, section-by-section) and a holography approach (to enhance phase resolution and phase coupling in imaging). Certain embodiments provide near field, ultrasonic holography, near field microwave holography, or other near field acoustic holography for surface and subsurface imaging in nano- and micro-specimens, such as biological, mechanical, and electrical specimens. Certain embodiments allow SNFUH imaging using linear and/or non-linear interactions between cantilever and specimen in contact, soft contact and/or near contact mode, for example.

As a result, the technique allows subsurface flaw imaging in nano- and micro-composites, MEMS, CMOS, and heterostructures, for example. The technique also provides in-vitro imaging of biopolymer, biomaterials and biological structures (e.g. viewing cell-membrane or implant-bio interface). Additionally, certain embodiments detect voiding and subsurface defects in low-K dielectric materials and interconnects, as well as stress migration and defect analysis in 3D interconnects and MEMS. Certain embodiments facilitate dopant profiling and modulus mapping in non-contact mode and also provide non-invasive monitoring of molecular markers/tags—signal pathways, for example.

In an embodiment, a high frequency (e.g., on the order of hundreds of MHz) acoustic wave is launched from the bottom of the specimen, while another wave is launched on the AFM cantilever. These acoustic waves are mixed together through a SNFUH electronic module, which includes a combination of various filters, mixers, feedback electronics and electronic components used to obtain a desired product and addition of fundamental resonances and related harmonics (in addition to difference frequencies). The resulting mixed wave is monitored by the AFM tip, which itself acts as an antenna for both phase and amplitude. As the specimen acoustic wave gets perturbed by buried features, especially its phase, the local surface acoustic waves are very effectively monitored by the AFM tip. Thus, within the near-field regime (which enjoys superb lateral and vertical resolution), the acoustic wave (which is non-destructive and sensitive to mechanical/elastic variation in its “path”) is fully analyzed, point-by-point, by the AFM acoustic antenna in terms of phase and amplitude. Thus, as the specimen is scanned across, a pictorial representation of acoustic wave's perturbation is fully recorded and displayed, to offer a “quantitative” account of internal microstructure of the specimen.

The SNFUH system is operational in the linear and near-contact regime of tip-sample interaction and proves effective for in-vitro imaging of biological cells and tissues using the SNFUH electronic module, for example.

Thus, certain embodiments provide an electronic readout based on an embedded MOSFET to detect product frequencies, which is thereby not limited by an optical detector. Moreover, electronic readout may help in building a parallel SNFUH system for industrial application. In addition, a Brillion Zone Scattering technique may be used to map the modules of any surface in non-destructive way with greater efficiency than other methods.

Certain embodiments may be applied to microelectronics, especially as an advanced nanoscale surface and sub-surface metrology tool-set. Further, certain embodiments provide imaging for Nanoelectronics, reliability and failure analysis in Microsystems (MEMS), and Nanotechnology, in general, and especially biomolecular interconnects and BioMEMS. Additionally, certain embodiments provide in-vitro imaging of biological structures without having to “open-up” internal structures. By combining the nanometer-scale spatial resolution of conventional SPMs with the sub-surface imaging capabilities, certain embodiments may characterize the surface defects and structures with high resolution and will have further potential for developing nanoscale non-invasive 3D tomography, for example.

Scanning Near Field Ultrasound Holography (SNFUH) may be used, for example, in near contact and contact mode with product frequencies for the following structures and devices: (1) Investigating mechanical uniformity and process-induced mechanical modification of materials in integrated circuit (IC) structures and MEMS; (2) Real-time in-vitro biological imaging of red blood cells infected with malaria parasites; (3) Voiding in copper interconnects and (4) Non-invasive monitoring of nanoparticles buried under polymeric films. Such capabilities may complement cross-sectional imaging techniques such as SEM-EDS (scanning electron microscope-energy dispersive spectroscopy), TEM-EDS (transmission electron microscope-energy dispersive spectroscopy), TEM-EELS (transmission electron microscope-electron energy-loss microscopy), and ex situ STM (scanning tunneling microscopy) to investigate the nanomechanics and subsurface imaging of material interfaces, the uniformity of conformally deposited coatings, and mechanical defects in multilayer structures, for example.

Characterization tools and techniques have been developed recently ranging from local surface probes such as STM/AFM to those which see the “inner” space such as, for example, synchrotron x-ray and spallation neutron facilities. However, despite these advances, a challenge in non-destructive imaging remains between the two ranges of length-scales offered by confocal/multi-photon techniques and x-ray/neutron imaging. As materials, structures and phenomena continue to “shrink,” as the micro/nanofabrication paradigms move from “planar” to “3-D/stacked” platforms and as micro electrical mechanical systems (MEMS) become more complicated and pervasive, there is an acute need to image and analyze buried defects in structures in multilayer structures and devices with high resolution/sensitivity in a rapid and non-destructive fashion.

Imaging with non-invasive radiations such as, for example, light and acoustic waves suffers from classical diffraction limit in attempting to attain sub-micrometer resolution. K. Inagaki, G. A. D. Briggs and O. B. Wright, Waveguide ultrasonic force microscopy at 60 MHz, App. Phys. Lett., Vol. 76 (2000) 1836; G. A. D. Briggs, Acoustic Microscopy, (Clarendon Press, Oxford, 1992) 33; T. M. Nelson, R. W. Smith, Scanning Acoustic microscopy; Advanced Materials and Processes, Vol. 162 (2004) 29-32. Scanning probe microscopy (SPM) and its analogs offer improved spatial resolution, but are sensitive only to surface or shallow surface features. C. F. Quate, A. Atalar, and H. K. Wickramasinghe, “Acoustic Microscopy with Mechanical Scanning—A Review,” Proceedings of the IEEE, 67 (8) (August 1979) 1092-1114; C. F. Quate, Invited Paper, “The AFM as a Tool for Surface Imaging,” Surface Science: The First Thirty Years, Vol. 299-300 (a Special Commemorative Volume of the 30th Birthday of the Journal Surface Science), Ed: Charles B. Duke, 980-995 (Elsevier Science Publishers, Amsterdam, January 1994); P. A. Weiss, Review of Scanning Probe Microscopy and Spectroscopy: Theory, Techniques and Applications, Jour. of American Chemical Society, Vol. 123 (2001) 9725. Near-field scanning optical microscopy (NSOM) offers sub-wavelength resolution, but it is unable to image optically opaque and deeply buried structures. W. Dickson, S. Takahashi, R. Polland, and R. Atkinson, High resolution optical imaging of magnetic-domain structures, IEEE Transactions on Nanotechnology, Vol. 4 (2005) 229; M. Denyer, R. Micheletto, K. Nakajima, M. Hara, Journal of Nanoscience and Nanotechnology, Vol. 3 (2003) 496.

Thus, with respect to “non-destructive” real-space imaging, there is a clear void between the two ranges of length-scales offered by confocal/photon or acoustic/sonography techniques (e.g., micrometer scale). W. Dickson, S. Takahashi, R. Polland, and R. Atkinson, High resolution optical imaging of magnetic-domain structures, IEEE Transactions on Nanotechnology, Vol. 4 (2005) 229; Dao-Zhong Huang, J. Bing Lin and Z. Sheng, Ultrasonographic characteristics of breast infiltrative ductal carcinoma. Chinese Journal of Medical Imaging Technology, Vol. 20 (2004) 1815-17; X. Wang, Y. Pang, G. Stoica and V. Wang, Three-dimensional laser-induced photo acoustic tomography of mouse-brain with the skin and skull intact, Optical Letters, Vol. 28 (2003) 1739; K. Kostli and Paul C. Beard, Two-dimensional photo acoustic imaging by use of fourier transform image reconstruction and detector with an anisotropic response, Applied Optics, Vol. 42 (2003) 1899. This is particularly true if the features of interest are buried deeper into the material, beyond the interaction range of proximal probes.

Numerous SPM related techniques to measure surface nanomechanical variations have been introduced with limited reproducibility and limited ability to probe deeply buried or embedded features or quantitative extraction of nanomechanical contrast. Ultrasonic force microscopy (UFM) (see, e.g., Bruce Altemus, G. S. Shekhawat, Robert Geer, Bai Xu and J. Castrace, Nanoscale elastic imaging of Micro Mechanical System based Micro Mirror, Proceeding of SPIE, Vol. 4558 (2001); R. E. Geer, O. V. Kolosov, G. A. D. Briggs, and G. S. Shekhawat, Nanometer-scale mechanical imaging of aluminum damascene interconnect structures in a low dielectric-constant, J. Appl. Phys. Vol. 91 (2002) 4549; O. V. Kolosov, M. Castell, Chris D Marsh and G. A. D. Briggs, Imaging the elastic nanostructures of Ge Islands by Ultrasonic Force Microscopy, Phys. Rev. Lett., Vol. 81 (1998) 1046; D. C. Hurley, K. Shen, N. N. Jennett and J. A. Turner, Atomic force acoustic microscopy methods to determine thin-film elastic properties, Journal of Applied Physics, Vol. 94 (2003) 2347; Hirotsugu Ogi, Jiayong Tian, Toyokazu Tada and Masahiko Hirao, Elastic stiffness mapping by resonance ultrasound microscopy with isolated piezoelectric oscillator, Appl. Phys. Lett., Vol. 83 (20030) 464-6; L. Muthuswami and R. E. Geer, Nanomechanical defect imaging in premetal dielectrics for integrated circuits, Appl. Phys. Lett., Vol 84 (2004) 5082; Y. Zheng, B. Sankaran and R. Geer, Proceedings of the SPIE—The International Society for Optical Engineering, Vol. 5766 (2005) 99; Tetard, L., Passian, A., Lynch, R., Shekhawat, G, Dravid, V. and Thundat, T., Appl. Phys. Lettes 93, 133113 (2008)) developed for microelectronics metrology by Dr. Shekhawat and others has enjoyed some success for nanomechanical mapping of elastic and viscoelastic properties of soft and hard surfaces. However, a wider deployment of this technique is generally marred by a lack of reproducibility, unpredictable environmental effects in the usual contact mode of imaging, and a lack of compelling evidence for demonstrated sensitivity to buried and embedded structure.

To overcome one or more of these limitations, some embodiments of the present invention employ a non-destructive metrology approach called scanning near-field ultrasound holography (SNFUH) (see, e.g., Tetard, L., Passian, A., Venmar, K., Lynch, R., Shekhawat, G., Dravid, V. and Thundat, T., Nature Nanotechnology, Vol. 3, 501, August 2008; Shekhawat, G. S. and Dravid, Vinayak, Nanaoscale imaging of buried nanostructures using Scanning Near-field ultrasound holography, Science, Vol. 310, 89 (2005); Cuberes, M. T., Alexander, H. E., Briggs, G. A. D. and Kolosov, O. V., J. Phys. D 33, 2347 (2000); G. S. Shekhawat, A. Srivastava, S. Avasthy and V. P. Dravid, Ultrasound holography for noninvasive imaging of buried defects in advanced interconnect architectures, Applied Physics Letters 95, 263101 (2009); Shekhawat, Gajendra and Dravid, V. P. “Scanning Near Field Ultrasound Holography,” U.S. Pat. No. 7,448,269) and demonstrated its potential for identifying buried features at high (e.g., 15-20 nm lateral) resolution, both in hard (e.g., semiconducting) samples and in soft (e.g., biological) samples. In SNFUH, a high frequency acoustic wave (e.g., of the order of a megahertz or higher and significantly more than the resonance frequency of a typical cantilever, f₀˜10-100 KHz) is launched from the bottom of the specimen, while another wave is launched on a SPM cantilever, albeit at a slightly different frequency. The interference of these two waves would nominally form surface acoustic waves, which is analogous to, for example, x-ray standing waves (XSW) that result from interference of scattered and reference x-ray waves. The perturbations to the phase and amplitude of the surface acoustic wave are locally monitored by the SPM acoustic antenna. As the specimen acoustic wave is perturbed by buried features, the resultant alteration in the surface acoustic wave, especially its phase, can be experimentally extracted from the tip deflection signal via a lock-in detection method. Thus, within the near-field regime, which may have superb spatial resolution, the acoustic wave, which is non-destructive and sensitive to mechanical and/or elastic variation along its path, is fully analyzed point-by-point by the SPM acoustic antenna in terms of its phase and amplitude. As the specimen is scanned across, a pictorial representation of the perturbation of the acoustic wave is fully recorded and displayed, offering a “quantitative” account of the internal microstructure of the specimen.

Despite its advantage over other acoustic methods, some of the possible limitations of this technique include:

-   -   Limited depth as well as spatial resolution due to diffraction         of sound waves     -   Requires very high frequency acoustics transducers to minimize         the diffraction effects, but custom fabrication of these high         frequency transducers is very difficult process (>100 MHz).         Moreover, as the frequency increases, the thickness of the         transducer goes down significantly and the sample under         investigation should not be thicker than the transducer         thickness. Significant damping of the acoustic amplitude will         happen if sample thickness is greater than the transducer         thickness.     -   Limitations in terms of acoustic coupling. Current SNFUH system         uses organic couplers that pose serious roadblocks to use it as         a sub-surface metrology toolset in semiconductor industry due to         contamination issues.     -   Acoustic couplings at wafer scale cannot be possible with         organic couplers. No other alternative is available which makes         this method not friendly for in-line inspection of buried         defects in semiconductor foundries.     -   Phase contrast in SNFUH can be suddenly enhanced with dominant         topographical effects when beat frequency approaches the contact         resonance of the cantilever. The topographic effect can make the         interpretation of data in the phase image difficult. The         topographical effect is due to a shift in contact resonance         caused by variation in surface elastic properties. The         topographical effect can be even more pronounced at higher         excitation frequencies (e.g., f_(c)=150 MHz) where the         cantilever more easily gets off from its resonance.     -   Organic ultrasonic coupling can pose a serious contamination         problem during imaging in aqueous ambient.

Some embodiments according to the present invention provide a laser or other light source that provides a coherent laser beam that is focused on a surface of a sample as illustrated in FIG. 10. The laser can be adjusted so that the frequency of the coherent laser beam can be selected in a range from approximately a MHz to approximately a GHz. The absorption of the beam radiation into the sample leads to a thermally induced expansion and results in the subsequent excitation of acoustic waves which can travel across the sample.

Some embodiments according to the present invention provide that the laser beam be modulated by a modulated waveform with a lower frequency. For example, the modulating waveform may have a frequency in a range from approximately 20 KHz to approximately 200 KHz.

The perturbations in the surface thermo acoustic waves can then be monitored locally in real-time with a high resolution SPM probe. The SPM probe may include a low frequency silicon nitride (SiN) and/or silicon cantilever. The SPM cantilever may have lower resonance frequencies, for example, from approximately 10 KHz to approximately 200 KHz. The SPM cantilever with lower resonance frequencies can easily detect the low frequency surface acoustic waves coming both from surface as well as subsurface features of the sample. The surface of the sample and/or buried defects inside of the sample can be detected with very high spatial resolution.

Some embodiments according to the present invention provide that acoustic coupling is not required.

Some embodiment according to the present invention provide that frequency can be easily varied from approximately a MHz to approximately a GHz with same source, for example, to minimize the diffraction effects and to achieve high resolution. FIG. 10 is a schematic illustration of laser generated surface ultrasonic waves generated by an SPM probe. FIG. 10 may illustrate, for example, amplitude detection.

As depicted in FIG. 10, the thermal waves resulting from the absorption of the intensity-modulated laser beam reflect and scatter from the surface as well as the sub-surface thermal features, thereby providing an image of these features. Thermal waves can be critically damped and difficult to image using far-field optics. Since these thermal waves trigger the thermoelastic acoustic waves, which have a same frequency as the thermal waves, but are normal propagating waves with little attenuation over fairly long distances, the SPM probe being in the near-field proximity can easily detect these perturbations to measure the nanomechanical properties of surface as well as sub-surface features. Thus, in scanning near field thermoelastic acoustic holography (SNFTAH), the interaction of the thermo elastically generated acoustic waves with the elastic features generates the image contrast.

Some embodiments according to the present invention provide a laser or other light source that provides a coherent laser beam that is focused on a surface of a sample as illustrated in FIG. 11. The laser can be adjusted so that the frequency of the coherent laser beam can be selected in a range from approximately a MHz to approximately a GHz. The absorption of the beam radiation into the sample leads to a thermally induced expansion and results in the subsequent excitation of acoustic waves which can travel across the sample.

Some embodiments according to the present invention provide that the laser beam be modulated by a modulated waveform with a lower frequency. For example, the modulating waveform may have a frequency in a range from approximately 20 KHz to approximately 200 KHz.

Some embodiments according to the present invention provide that a coherent laser beam (e.g., in a range from approximately a MHz to approximately a GHz) is modulated with 20-200 KHz waveform on the top of the carrier wave (e.g., in a range from approximately a MHz to approximately a GHz) and that thermo acoustic waves are generated from the sample surface.

Some embodiments according to the present invention provide that the SPM probe is oscillated at a frequency in the range of approximately 20 KHz to approximately 200 KHz, for example. The frequency at which SPM probe oscillates, the frequency of the modulating waveform and/or the frequency of the laser beam can be adjusted, selected and/or programmed. Some embodiments according to the present invention may provide that more than one frequency can be used for each of the frequency at which the SPM probe oscillates, the frequency of the modulating waveform and/or the frequency of the laser beam. In addition, some embodiments according to the present invention contemplate using fundamental and/or harmonic frequencies, frequency additions and/or differences, frequency beats and/or frequency products.

The interference of the low frequency thermo elastically generated acoustic waves in the sample and the cantilever waveforms would nominally form surface acoustic wave (e.g., beats), which may be analogous to, for example, x-ray standing waves (XSW) that result from interference of scattered and reference x-ray waves. The perturbations to the phase and amplitude of the surface acoustic wave are locally monitored by the SPM acoustic antenna.

As the specimen acoustic wave is perturbed by buried features, the resultant alteration in the surface acoustic wave, especially its phase, can be experimentally extracted from the tip deflection signal via a lock-in detection method (e.g., a conventional lock-in detection method). Thus, within the near-field regime, which may enjoy superb spatial resolution, the acoustic wave, which is non-destructive and sensitive to mechanical and/or elastic variation along its path, is fully analyzed, point-by-point, by the SPM acoustic antenna in terms of its phase and amplitude. As the specimen is scanned across, a pictorial representation of acoustic wave's perturbation is fully recorded and displayed, offering a “quantitative” account of the internal microstructure of the specimen. Diffraction effects can be reduced and/or eliminated by using the high frequency laser excitation. As a result, greater depth as well as spatial resolution can be achieved. Amplitude and/or phase signals are detected using SNFTAH.

FIG. 11 shows of an embodiment according to the present invention. In particular, FIG. 11 is a schematic illustration of laser generated surface ultrasonic waves generated by a SPM probe. FIG. 11 may also illustrate amplitude and/or phase SNFTAH detection. For example, acoustic phase detection may provide the high resolution quantitative depth information of buried nanostructures. Some embodiments according to the present invention provide a projected depth resolution of approximately 1 nm at approximately 1 GHz excitation frequency.

Some embodiments according the present invention may be used in one or more of the following applications:

-   -   Sub-surface flaw detection     -   Buried defects, cracks and delaminations in semiconductor         interfaces and devices     -   In-line inspection tool in semiconductor industry for         nondestructive sub-surface defect and failure analysis     -   Quantitative mechanical analysis in low-K dielectric materials     -   Dopant detection     -   Stress migration in 3D M EMS and interconnects     -   Non-invasive imaging of sub-cellular features in biological         specimens     -   Quantitative phase measurements in multi-layer thin films and         structures

Many other applications of the present invention as well as modifications and variations are possible in light of the above teachings. While the present invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention not be limited to the particular embodiment disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims. 

1. An system for analyzing a sample, comprising: a light source that generates a coherent laser beam, wherein the coherent laser beam is modulated by a waveform of a lower frequency, wherein the modulated laser beam is absorbed by the sample causing thermally induced expansion and resulting in an excitation of acoustic waves; and a scanning probe microscope probe is locally deployed near the sample, wherein the probe detects perturbations in the excited acoustic waves in real time to detect surface and buried structures of the sample.
 2. The system of claim 1, wherein the light source is configured to generate a coherent laser beam of adaptable frequency.
 3. The system of claim 1, wherein the light source is configured to generate a coherent laser beam of adaptable frequency, and wherein the adaptable frequency is selectable in a range from approximately a megahertz to approximately a gigahertz.
 4. The system of claim 1, wherein the coherent laser beam is configured to a select an adaptable frequency in a range from approximately a megahertz to approximately a gigahertz, and wherein the selected frequency minimizes diffraction effects.
 5. The system of claim 4, wherein the lower frequency waveform is in a range of approximately 2 kilohertz to approximately 200 kilohertz.
 6. The system of claim 1, wherein the probe comprises silicon nitride or silicon cantilever.
 7. The system of claim 1, wherein the system employs near-field holography.
 8. The system of claim 1, wherein the probe comprises a cantilever that is characterized by resonance frequencies in a range of approximately 10 kilohertz to approximately 200 kilohertz.
 9. The system of claim 1, wherein the acoustic waves comprise thermoelastic acoustic waves that have a same frequency as thermal waves caused by the absorption of the modulated laser beam.
 10. The system of claim 1, wherein the probe is in near-field proximity.
 11. The system of claim 1, wherein the system does not use far-field optics to image the acoustic waves.
 12. The system of claim 1, wherein an interaction between the acoustic waves and elastic features of the sample generates image contrast.
 13. A method for analyzing a sample, comprising: providing a coherent laser beam at a first frequency; modulating the coherent laser beam with a second frequency, the second frequency being lower than the first frequency; absorbing the modulated laser beam by the sample; generating thermo acoustic waves from the absorbed laser beam; oscillating a scanning probe at a third frequency; and detecting acoustic phase of surface acoustic waves formed by interference between oscillations of the scanning probe and thermo acoustic waves generated from the absorbed laser beam.
 14. The method of claim 13, wherein the second frequency is in a range of approximately 20 kilohertz to approximately 200 kilohertz, and wherein the third frequency is in a range of approximately 20 kilohertz to approximately 200 kilohertz.
 15. The method of claim 13, wherein the first frequency is adjustable in a range of approximately a megahertz to approximately 1 gigahertz, and wherein the method is used by a system that uses near-field holography.
 16. The method of claim 15, wherein the first frequency is set to minimize diffraction effects.
 17. The method of claim 13, comprising monitoring, by an acoustic antenna, perturbations to phase and amplitude of surface acoustic waves formed by interference between oscillations of the scanning probe and thermo acoustic waves generated from the absorbed laser beam.
 18. The method of claim 13, comprising providing high resolution quantitative depth information of buried nanostructures based on at least the detected acoustic phase.
 19. The method of claim 18, wherein the first frequency is approximately 1 gigahertz, and wherein the high resolution quantitative depth information has a resolution of approximately 1 nanometer.
 20. The method of claim 13, wherein the first frequency, the second frequency or the third frequency comprises harmonic frequencies. 